The present invention relates to electrical isolators and in particular to a microelectromechanical system (MEMS) device providing electrical isolation in the transmission of analog electrical signals.
Electrical isolators are used to provide electrical isolation between circuit elements for the purposes of voltage level shifting, electrical noise reduction, and high voltage and current protection.
Circuit elements may be considered electrically isolated if there is no path in which a direct current (DC) can flow between them. Isolation of this kind can be obtained by capacitive or inductive coupling. In capacitive coupling, an electrical input signal is applied to one plate of a capacitor to transmit an electrostatic signal across an insulating dielectric to a second plate at which an output signal is developed. In inductive coupling, an electrical input signal is applied to a first coil to transmit an electromagnetic field across an insulating gap to a second coil, which generates the isolated output signal. Both such isolators essentially block steady state or DC electrical signals.
Such isolators, although simple, block the communication of signals that have significant low frequency components. Further, these isolators can introduce significant frequency dependent attenuation and phase distortion in the transmitted signal. These features make such isolators unsuitable for many types of signals including many types of high-speed digital communications.
In addition, it is sometimes desirable to provide high voltage ( greater than 2 kV) isolation between two different portions of a system, while maintaining a communication path between these two portions. This is often true in industrial control applications where it is desirable to isolate the sensor/actuator portions from the control portions of the overall system. It is also applicable to medical instrumentation systems, where it is desirable to isolate the patient from the voltages and currents within the instrumentation.
The isolation of digital signals is frequently provided by optical isolators. In an optical isolator, an input signal drives a light source, typically a light emitting diode (LED) positioned to transmit its light to a photodiode or phototransistor through an insulating but transparent separator. Such a system will readily transmit a binary signal of arbitrary frequency without the distortion and attenuation introduced by capacitors and inductors. The optical isolator further provides an inherent signal limiting in the output through saturation of the light receiver, and signal thresholding in the input, by virtue of the intrinsic LED forward bias voltage.
Nevertheless, optical isolators have some disadvantages. They require a relatively expensive gallium arsenide (GaAs) substrate that is incompatible with other types of integrated circuitry and thus optical isolators often require separate packaging and assembly from the circuits they are protecting. The characteristics of the LED and photodetector can be difficult to control during fabrication, increasing the costs if unit-to-unit variation cannot be tolerated. The power requirements of the LED may require signal conditioning of the input signal before an optical isolator can be used, imposing yet an additional cost. While the forward bias voltage of the LED provides an inherent noise thresholding, the threshold generally cannot be adjusted but is fixed by chemical properties of the LED materials. Accordingly, if different thresholds are required, additional signal conditioning may be needed. Finally, the LED is a diode and thus limits the input signal to a single polarity unless multiple LEDs are used.
It is common to process analog electrical signals using digital circuitry such as microprocessors. In such situations, the analog signal may be periodically sampled and the samples converted into digital words input by an analog to digital converter (A/D) to and processed by the digital circuitry. Conversely, digital words produced by the digital circuitry may be converted into an analog signal through the use of a digital-to-analog converter (D/A) to provide a series of analog electrical values that may be filtered into a continuous analog signal. Isolation of such signals at the interface to the digital circuitry is often desired and may be performed by placing an optical isolator in series with the electrical signal representing each bit of the relevant digital word after the A/D converter and before the DIA converter. Particularly in the area of industrial controls where many isolated analog signals must be processed and output, a large number of optical isolators are required rendering the isolation very costly or impractical.
The present invention provides a mechanical isolator manufactured using MEMS techniques and suitable for transmitting analog signals without prior conversion to digital signals. A special fabrication process forms a microscopic beam whose ends are insulated from each other. One end of the beam is connected to a microscopic actuator, which receives an analog input signal to move the beam in proportion to a generated actuator force. The other isolated end of the beam is attached to a sensor detecting movement of the beam to provide a corresponding analog value. The small scale of the total device provides inexpensive, fast and reliable response.
Specifically, the present invention provides a microelectromechanical system (MEMS) analog isolator having a substrate and an element supported from the substrate for continuous movement between a first and second position with respect to the substrate, where at least a portion of the element between a first and second location on the element is an electrical insulator to electrically isolate the first and second locations from each other. An actuator attached to the first portion of the element receives an input electrical signal and exerts a force dependent on the input electrical signal urging the element toward the second position. A control device attached to the element to exert a force dependent on the displacement of the element toward the first position and a sensor assembly communicating with the second portion of the element provide an analog output electrical signal dependent on movement of the element between the first position and the second position.
It is one object of the invention to produce a simple mechanical isolation system using MEMS techniques suitable for direct isolation of an analog signal overcoming the need for many optical isolators and further avoiding many of the disadvantages of current optical isolators in costs, interdevice consistency, and incompatibility with other integrated circuit components. In addition, the present invention requires no preconditioning of the input signal. The input voltage, current, or mechanical displacement can be applied directly to the device with no pre-processing.
The control element may be a spring or its equivalent and the sensor assembly may include a sensor providing the analog output electrical signal based on the amount of movement of the element.
Thus another object of the invention is to provide the possibility of a simple open-loop analog isolator where the analog signal is transmitted over an insulated beam by motion of the beam.
Alternatively, the control element may be a second actuator attached to the element to receive a feedback electrical signal and exert a force dependent on the feedback electrical signal urging the element toward the first position. In this case, the sensor assembly may include a sensor indicating a location of the element with respect to a null position and an error detector receiving the output electrical signal to generate the feedback electrical signal so as to tend to restore the element to the null position. The output electrical signal is derived from the feedback signal.
Another object of the invention is thus to permit a more complex analog isolator using feedback techniques where the analog signal is transmitted as forces permitting minimal movement of the beam thus avoiding mechanical non-linearities.
The control element may further include a third actuator attached to the element to receive a second feedback signal and exert a force dependent on the second feedback electrical signal urging the element toward the second position.
It is thus a further object of the invention to permit a feedback control of the beam allowing feedback signals that may exert either a force urging the element toward the first position or a force urging the element toward the second position.
The above described error detector may produce a binary electrical feedback signal indicating a position of the beam with respect to the null location between the first and second positions and further including a pulse width demodulator circuit evaluating the duty cycle of the feedback signal to produce the output electrical signal.
It is thus another object of the invention to provide a simple method of extracting a multi-bit digital signal from the isolator of the present invention. The duty cycle demodulator may be a simple counting circuit.
The actuator may be an electrostatic motor or a Lorenz-force motor or a piezoelectric motor or a thermal-expansion motor or a mechanical-displacement motor.
It is therefore another object of the invention to provide an isolator that may receive a variety of different electrical signals that may not be compatible with an optical isolator LED, for example, those having a voltage of less than 0.7 volts.
Similarly, the control element may be an electrostatic motor, a Lorenz-force motor, a piezoelectric motor, a thermal-expansion motor, a mechanical-displacement motor, or a mechanical spring.
Thus the invention may provide both for an extremely simple control element that requires no electrical connection (e.g. a mechanical spring) or an adjustable control element that allows the null point of the beam to be freely adjusted.
The sensor may be a capacitive sensor or a piezoelectric sensor or a photoelectric sensor or a resistive sensor or an optical switching sensor.
It is therefore another object of the invention to provide flexible variety of sensing techniques suitable for different purposes.
In one embodiment of the invention, the element may be a beam attached to the substrate for sliding motion between the first and second positions. The beam may be supported by flexing transverse arm pairs attached at longitudinally opposed ends of the beam to extend outward therefrom.
Thus it is another object of the invention to provide a simple mechanism that may be implemented on a microscopic scale using MEMS technologies for supporting the element for motion.
The flexing transverse arms may include a cantilevered first portion having first ends attached to the beam and second ends attached to an elbow portion removed from the beam and a cantilevered second portion substantially parallel to the first portion and having a first end attached to the substrate proximate to the beam and a second end attached to the elbow portion. Further the beam and the transverse arms may be symmetric across a longitudinal beam access.
Thus it is another object of the invention to provide a microscopic structure that is resistant to thermal expansion due to processing temperatures or to changes in the operating temperature. The symmetry ensures that the beam remains centered with thermal expansion while the doubling back of the flexible transverse arms provides for a degree of stress relief.
The flexing transverse arms may attach to the substrate through a spring section allowing angulations of the ends of the transverse arms with respect to the substrate.
It is thus another object of the invention to allow an effective pivoting of the flexible transverse arms so as to decrease the stiffness of the beam structure.
One embodiment of the invention may include a magnetic field, which may be produced by a magnet, crossing the beam and at least one flexing transverse arm may be conductive to an electrical signal and exert a force dependent on the electrical signal urging the beam toward a position.
It is thus another object of the invention to provide that the same structure used to support the beam may provide for its actuation or control.
The beam may include transverse extending primary capacitor plates attached to the beam and extending out from the beam proximate to secondary capacitor plates. The effective area of the primary capacitor plates may be equal across the longitudinal axis of the beam and the capacitor plates may be attached to the beam between attachment points of at least two of the flexing transverse arm pairs. In one embodiment, the capacitors may include interdigitated fingers. Parallel plate capacitors will also work (although they have less linearity).
Another object of the invention is to provide a method for the integration of an electrostatic motor to the isolator in a way that balanced and well-supported forces may be obtained.
The primary capacitor plates may be positioned with respect to the secondary capacitor plates so as to draw the primary capacitor plates toward the secondary capacitor plates on one side of the beam while to separate the primary capacitor plates from the secondary capacitor plates on the other side of the beam. Conversely, the capacitor plates may be positioned so that all draw together with a given motion.
Thus it is another object of the invention to allow the capacitor plates to be used as a sensor in which a comparison of capacitance values reveals a position of the beam or as an electrostatic motor.
The beam may include a first and second micro-machined layer, the first of which is insulating to provide the portion of the electrical insulator in a region where the second layer is removed.
Thus it is another object of the invention to provide a simple method for forming insulating and conductive elements required by the present invention.
The electrical insulator of the beam may be between the actuator and the control element or between the control element and the sensor or both.
It is further an object of the invention to provide that the controlling circuit may be placed on either side of the isolation or to provide redundant isolation for greater total isolation.
The analog isolator may include a second sensor at a first portion of the element to provide a second output electrical signal indicating movement of the element to the second position, the output second electrical signal being electrically isolated from the first output electrical signal.
Thus it is another object of the invention to provide for an isolator that produces a signal indicating movement of the beam and thus operation of the isolator from the isolated side.
The isolator may further include a second actuator as a second portion of the element to receive a second input signal and exert a force dependent on the second input electrical signal urging the element toward the second position.
Thus it is another object of the invention to provide for a bi-directional electrical isolator suitable for use in multi-level control loops or for the purpose of resetting a scaling factor.
The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.